Through-The-Wall Differential Distance Tracking With Ultra-High Repetition Rate Terahertz Time-Domain Spectroscopy

Terahertz time-domain spectroscopy systems driven by monolithic mode-locked laser diodes provide a high dynamic range across a bandwidth of several hundred gigahertz in a compact and robust device, making them useful for mobile distance measurement with sub-millimeter resolution. However, their unambiguous range is limited to a few millimeters due to their ultra-high pulse repetition rate of a few ten gigahertz. In this work, we propose a differential measurement approach for tracking the distance to a slowly accelerated object with a virtually unlimited unambiguous range. We demonstrate distance tracking through a 12.5mm thick sheet of drywall with an accuracy of 78 $\mu$m.